Film-stretching apparatus and method of producing film

ABSTRACT

A film-stretching apparatus in accordance with an embodiment of the present invention includes a stretching furnace having (i) an entrance through which a material film is transferred into the stretching furnace and (ii) an exit through which the material film having been stretched is transferred out of the stretching furnace. An airflow in the entrance is controlled so as to be directed toward an inside of the stretching furnace. A gas in the stretching furnace is controlled so as to be discharged outside the stretching furnace.

This Nonprovisional application claims priority under 35 U.S.C. § 119 on Patent Application No. 2016-183180 filed in Japan on Sep. 20, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to (i) a film-stretching apparatus and (ii) a method of producing a film.

BACKGROUND ART

As a separator for use in a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, there has been used (i) a microporous film which contains a polyolefin as its main component or (ii) a laminated porous film obtained by stacking a functional layer on the microporous film which serves as a base material.

In a process of producing such a film, the film is longitudinally or transversely stretched so as to control a pore structure of the film. Patent Literature 1 discloses that a polyolefin microporous film suitable for a base material porous film serving as a separator can be produced with high productivity, by causing a plurality of stretching regions in a stretching furnace of a film-stretching apparatus to have respective different film widening speeds and respective different temperatures such that the film widening speeds and the temperatures are in a specific relationship.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication Tokukai No. 2013-159750 (Publication date: Aug. 19, 2013)

SUMMARY OF INVENTION Technical Problem

According to the above conventional technique, however, since a pressure in the stretching furnace is set to be equal to an atmospheric pressure, it is likely that a volatile component having been volatilized by heating a film leaks out of the stretching furnace. The volatile component having leaked out of the stretching furnace is condensed and deposited outside the stretching furnace, and drops or falls on the film. This may cause damage to the film.

The present invention has been made in view of the above problem, and an object of the present invention is to realize (i) a film-stretching apparatus and (ii) a method of producing a film, each of which allows a reduction in damage to a film which damage is caused by leakage of a volatile component from a stretching furnace.

Solution to Problem

In order to attain the above object, a film-stretching apparatus in accordance with an aspect of the present invention is a film-stretching apparatus for stretching a film, including: a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, an airflow in the entrance being controlled so as to be directed toward an inside of the stretching furnace, a gas in the stretching furnace being controlled so as to be discharged outside the stretching furnace.

In order to attain the above object, a film-stretching apparatus in accordance with an aspect of the present invention is a film-stretching apparatus for stretching a film, including: a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, an amount of air to be discharged from at least part of the stretching furnace being controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace, the at least part of the stretching furnace extending from the entrance to a location at which the film starts to be stretched.

In order to attain the above object, a method of producing a film in accordance with an aspect of the present invention includes: a film stretching step of stretching a film, the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, in the film stretching step, an airflow in the entrance being controlled so as to be directed toward an inside of the stretching furnace, and a gas in the stretching furnace being controlled so as to be discharged outside the stretching furnace.

In order to attain the above object, a method of producing a film in accordance with the present invention includes: a film stretching step of stretching a film, the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, in the film stretching step, an amount of air to be discharged from at least part of the stretching furnace being controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace, the at least part of the stretching furnace extending from the entrance to a location at which the film starts to be stretched.

Advantageous Effects of Invention

According to an embodiment of the present invention, it is possible to provide (i) a film-stretching apparatus and (ii) a method of producing a film, each of which allows a reduction in damage to a film which damage is caused by leakage of a volatile component from a stretching furnace.

Particularly, for the following reasons, an embodiment of the present invention can be suitably applied to a case where a polyolefin film, to be used as a separator for a nonaqueous electrolyte secondary battery, is stretched.

A polyolefin film is rarely used alone as a separator, and, as will be later described, a functional layer is often stacked on the polyolefin film by applying a coating material to the polyolefin film. In a case where (i) a volatile component having been volatilized from the polyolefin film is condensed and deposited and (ii) the volatile component thus condensed and deposited causes damage to part of the polyolefin film, that part of the polyolefin film varies in wettability. This causes the part of the polyolefin film to repel the coating material. In order that the polyolefin film is prevented from repelling the coating material and a functional layer (i.e., a coating layer) is uniformly formed on the polyolefin film, an embodiment of the present invention can be suitably applied to a case where the polyolefin film is stretched.

A separator has a porous structure so as to allow lithium ions to move between a cathode and an anode while causing the cathode and the anode to be separated (later described). In a case where (i) the volatile component having been volatilized from the polyolefin film is condensed and deposited and (ii) the volatile component thus condensed and deposited causes damage to part of the polyolefin film, that part of the polyolefin film cannot function as a separator because pores in the part of the polyolefin film are blocked by such a condensed and deposited volatile component. In order that such partial blocking of pores in the polyolefin film is prevented, an embodiment of the present invention can be suitably applied to a case where the polyolefin film is stretched.

Moreover, in a case where a liquid plasticizing agent is employed as a pore forming agent in production of a separator, the polyolefin film is stretched in a state where the polyolefin film contains a large amount of the plasticizing agent. In a case where the plasticizing agent having been volatilized from the polyolefin film is condensed and drops on the polyolefin film, a polymer density locally decreases in part of the polyolefin film on which part the plasticizing agent has dropped. This may cause (i) that part of the polyolefin film to be different in pore structure from the other part of the polyolefin film and/or (ii) that part of the polyolefin film to have a pinhole. In order that the polyolefin film has a uniform pore structure, an embodiment of the present invention can be suitably applied to a case where the polyolefin film is stretched.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a cross-sectional configuration of a lithium ion secondary battery.

Each of FIGS. 2A-2C is a diagram schematically illustrating a state of the lithium ion secondary battery illustrated in FIG. 1.

Each of FIGS. 3A-3B is a diagram schematically illustrating a state of a lithium ion secondary battery having another configuration.

FIG. 4 is a flowchart schematically showing a method of producing an original sheet of a separator.

Each of FIGS. 5A-5B is a cross-sectional view illustrating how a stretching step is carried out by use of a film-stretching apparatus.

FIG. 6 is a cross-sectional view illustrating a front chamber of a stretching furnace included in the film-stretching apparatus.

FIG. 7 is a cross-sectional view illustrating a pre-heating chamber of the stretching furnace.

FIG. 8 is a graph showing (i) an amount of air (air supply amount), per unit volume, supplied to each chamber of the stretching furnace and (ii) an amount of air (air discharge amount), per unit volume, discharged from each chamber of the stretching furnace.

FIG. 9A is a cross-sectional view illustrating an airflow in an entrance of the stretching furnace. FIG. 9B is a cross-sectional view illustrating an airflow in an exit of the stretching furnace.

FIG. 10 is a flowchart schematically showing a method of producing a separator.

Each of FIGS. 11A-11B is a diagram schematically illustrating an example of a slitting step.

DESCRIPTION OF EMBODIMENTS Embodiment 1

The following description will discuss an embodiment of the present invention with reference to FIGS. 1 through 9B. In Embodiment 1, an example case will be described in which a film-stretching apparatus in accordance with an embodiment of the present invention is applied to production of an original sheet of a separator (hereinafter referred to as a “separator original sheet”) which original sheet serves as a base material of a separator for a lithium ion secondary battery (hereinafter, sometimes referred to as a “separator”).

First, the lithium ion secondary battery will be described below with reference to FIGS. 1 through 3B.

(Configuration of Lithium Ion Secondary Battery)

A nonaqueous electrolyte secondary battery, typified by a lithium ion secondary battery, has a high energy density, and is therefore currently widely used as (i) batteries for use in devices such as personal computers, mobile phones, and mobile information terminals, (ii) batteries for use in moving bodies such as automobiles and airplanes, and (iii) stationary batteries contributing to stable power supply.

FIG. 1 is a diagram schematically illustrating a cross-sectional configuration of a lithium ion secondary battery 1. As illustrated in FIG. 1, the lithium ion secondary battery 1 includes a cathode 11, a separator 12, and an anode 13. An external device 2 is provided outside the lithium ion secondary battery 1 such that the external device 2 is connected to the cathode 11 and the anode 13. While the lithium ion secondary battery 1 is being charged, electrons move in a direction A. On the other hand, while the lithium ion secondary battery 1 is being discharged, the electrons move in a direction B.

(Separator)

The separator (film) 12 is provided so as to be sandwiched between the cathode 11, which is a positive electrode of the lithium ion secondary battery 1, and the anode 13, which is a negative electrode of the lithium ion secondary battery 1. The separator 12 causes the cathode 11 and the anode 13 to be separated, while allowing lithium ions to move between the cathode 11 and the anode 13. For example, a polyolefin such as polyethylene or polypropylene is used as a base material of the separator 12.

Each of FIGS. 2A-2C schematically illustrates a state of the lithium ion secondary battery 1 illustrated in FIG. 1. FIG. 2A illustrates a normal state. FIG. 2B illustrates a state in which a temperature of the lithium ion secondary battery 1 has risen. FIG. 2C illustrates a state in which the temperature of the lithium ion secondary battery 1 has sharply risen.

As illustrated in FIG. 2A, the separator 12 is provided with many pores P. Normally, lithium ions 3 in the lithium ion secondary battery 1 can move back and forth through the pores P.

Here, the temperature of the lithium ion secondary battery 1 may rise due to, for example, excessive charging of the lithium ion secondary battery 1 or a high electric current caused by short-circuiting of the external device. In such cases, the separator 12 melts or softens, so that the pores P are blocked as illustrated in FIG. 2B. As a result, the separator 12 shrinks. This stops back-and-forth movement of the lithium ions 3, and consequently stops the above-described temperature rise.

However, in a case where the temperature of the lithium ion secondary battery 1 sharply rises, the separator 12 suddenly shrinks. In this case, as illustrated in FIG. 2C, the separator 12 may be destroyed. This causes the lithium ions 3 to leak out from the separator 12 which has been destroyed. As a result, the lithium ions 3 do not stop moving back and forth. Consequently, the temperature continues rising.

(Heat-Resistant Separator)

Each of FIGS. 3A-3B schematically illustrates a state of the lithium ion secondary battery 1 having another configuration. FIG. 3A illustrates a normal state, and FIG. 3B illustrates a state in which the temperature of the lithium ion secondary battery 1 has sharply risen.

As illustrated in FIG. 3A, the lithium ion secondary battery 1 can further include a heat-resistant layer (functional layer) 4. The heat-resistant layer 4 can be provided on the separator 12. FIG. 3A illustrates a configuration in which the heat-resistant layer 4 serving as a functional layer is provided on the separator 12. A film in which the heat-resistant layer 4 is provided on the separator 12 is hereinafter referred to as a heat-resistant separator (film) 12 a.

In the configuration illustrated in FIG. 3A, the heat-resistant layer 4 is stacked on one of surfaces of the separator 12 which one is located on a cathode 11 side. Note that the heat-resistant layer 4 can alternatively be stacked on the other one of the surfaces of the separator 12 which other one is located on an anode 13 side or can alternatively be stacked on each of the surfaces of the separator 12. The heat-resistant layer 4 is also provided with pores which are similar to the pores P. Normally, the lithium ions 3 move back and forth through (i) the pores P and (ii) the pores of the heat-resistant layer 4. The heat-resistant layer 4 contains, for example, a wholly aromatic polyamide (aramid resin) as its material.

As illustrated in FIG. 3B, even in a case where the temperature of the lithium ion secondary battery 1 sharply rises and, as a result, the separator 12 melts or softens, a shape of the separator 12 is maintained because the heat-resistant layer 4 supports the separator 12. This causes the separator 12 to come off with melting or softening. Consequently, the pores P are only blocked up. This stops the back-and-forth movement of the lithium ions 3, and consequently stops the above-described excessive discharging or excessive charging. In this way, the separator 12 is prevented from being destroyed.

(Method of Producing Separator Original Sheet)

Next, a method of producing a separator original sheet which is to be used as a base material of a separator (film production method) will be described below with reference to FIG. 4. In Embodiment 1, an example case will be described in which the separator original sheet contains polyethylene as its main material.

FIG. 4 is a flowchart schematically showing a method of producing a separator original sheet 12 c. The following flow of producing the separator original sheet 12 c is exemplified by a method in which (i) a solid or liquid pore forming agent is added to a thermoplastic resin, (ii) a resultant resin composition is formed into a film, and then (iii) the pore forming agent is removed by use of an appropriate solvent. Specifically, in a case where the separator original sheet 12 c is made of a polyethylene resin which contains ultra-high molecular weight polyethylene, the flow of producing the separator original sheet 12 c includes a kneading step S1, a sheet-forming step S2, a removing step S3, and a stretching step S4 which are carried out in this order. Note that the removing step S3 and the stretching step S4 can be alternatively carried out in reversed order.

The kneading step S1 is a step of kneading ultra-high molecular weight polyethylene and a pore forming agent, such as calcium carbonate, so as to obtain a polyethylene resin composition. In the kneading step S1, for example, (i) a pore forming agent is added to an ultra-high molecular weight polyethylene powder, (ii) the ultra-high molecular weight polyethylene powder and the pore forming agent are mixed together, and (iii) a resultant mixture is melted and kneaded by use of, for example, a twin screw kneading extruder. A polyethylene resin composition is thus obtained.

The sheet-forming step S2 is a step of forming, into a film, the polyethylene resin composition obtained in the kneading step S1. In the sheet-forming step S2, for example, the polyethylene resin composition obtained in the kneading step S1 is rolled by use of a pair of rollers so that the polyethylene resin composition is formed into a film. A material film is thus obtained.

The removing step S3 is a step of removing the pore forming agent from the material film obtained in the sheet-forming step S2. In the removing step S3, for example, the material film is immersed in a hydrochloric acid aqueous solution or the like so as to dissolve the pore forming agent such as calcium carbonate. The pore forming agent is thus removed from the material film.

The stretching step S4 is a step of stretching the material film from which the pore forming agent has been removed in the removing step S3, so as to obtain a separator original sheet 12 c. The stretching step S4 is carried out by use of a film-stretching apparatus. The material film is stretched in a machine direction (i.e., a direction in which the material film is transferred) and a transverse direction (i.e., a width direction of the material film) while being transferred in a stretching furnace of the film-stretching apparatus. The stretching step S4 can further include a step of stretching the material film in the machine direction by use of a plurality of heated rollers. Note that the film-stretching apparatus will be later described in detail.

According to the above flow of producing the separator original sheet 12 c, many micropores are formed in the material film in the removing step S3. The micropores in the material film which has been stretched in the stretching step S4 serve as the respective pores P. The separator original sheet 12 c which is a polyethylene microporous film having a given thickness and a given air permeability is thus produced.

Note that the separator original sheet 12 c can be produced through a similar flow, even in a case where the separator original sheet 12 c contains another material. Note also that the method of producing the separator original sheet 12 c is not limited to the above method in which the pore forming agent is removed, and various methods can be alternatively employed to produce the separator original sheet 12 c.

(Film-Stretching Apparatus)

Next, a film-stretching apparatus 5 by use of which the stretching step (film-stretching step) S4 is carried out will be described below with reference to FIGS. 5 through 9B.

Each of FIGS. 5A-5B is a cross-sectional view illustrating how the stretching step S4 is carried out by use of the film-stretching apparatus 5. FIG. 5A illustrates a cross section of the film-stretching apparatus 5 which cross section is obtained by cutting the film-stretching apparatus 5 along a plane parallel to a machine direction MD and a transverse direction TD of a material film 12 d. FIG. 5A illustrates a cross section of the film-stretching apparatus 5 which cross section is obtained by cutting the film-stretching apparatus 5 along a plane perpendicular to the transverse direction TD.

The film-stretching apparatus 5 is a tenter-type stretching apparatus which stretches the material film 12 d by a tenter stretching method. The tenter-type stretching apparatus has such a mechanism that a plurality of chucks, which hold both sides of a film, move on tenter rails, which are continuously provided so as to extend from an entrance of a stretching furnace toward an exit of the stretching furnace, so that the film is uniaxially or biaxially stretched continuously.

The film-stretching apparatus 5 includes a stretching furnace 6 in which the material film 12 d is stretched. The stretching furnace 6 has (i) an entrance 61 through which the material film 12 d is transferred into the stretching furnace 6 and (ii) an exit 62 through which the material film 12 d having been stretched (i.e., the separator original sheet 12 c) is transferred out of the stretching furnace 6.

In the film-stretching apparatus 5, the material film 12 d having been transferred into the film-stretching apparatus 5 through the entrance 61 is stretched in the transverse direction (width direction) TD by the tenter stretching method while being transferred in the machine direction MD. A separator original sheet 12 c, which is obtained by thus stretching the material film 12 d, is transferred out of the film-stretching apparatus 5 through the exit 62.

The stretching furnace 6 is divided, along the machine direction MD of the material film 12 d, into a plurality of air amount control zones in each of which an amount of air is independently controllable. Specifically, the stretching furnace 6 includes a front chamber 6 a, a pre-heating chamber 6 b, a first stretching chamber 6 c, a second stretching chamber 6 d, a third stretching chamber 6 e, a heat-fixing chamber 6 f, and a back chamber 6 g which are located in this order from an entrance 61 side. In Embodiment 1, out of those chambers, each of the front chamber 6 a and the back chamber 6 g is configured such that an amount of air therein is independently controllable. Meanwhile, each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f is configured such that an amount of air and an operating temperature therein are independently controllable.

Alternatively, each of the front chamber 6 a, the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, the heat-fixing chamber 6 f, and the back chamber 6 g can be arranged such that an amount of air and an operating temperature therein are independently controllable.

Table 1 below shows (i) a function of each of the chambers (rooms), (ii) whether air is supplied to and/or discharged from each of the chambers, (iii) whether a film is stretched in each of the chambers, and (iv) an example of an operating temperature range in each of the chambers.

TABLE 1 Supply and Whether film Operating Name of discharge is transversely temperature room Function of air stretched range Front Prevention Discharge No No chamber of air only temperature leakage control Pre- Pre-heating Supply No 115-125 heating of film and chamber discharge First Transverse Supply Yes 100-115 stretching stretching and chamber discharge Second Transverse Supply Yes 100-115 stretching stretching and chamber discharge Third Transverse Supply Yes 100-115 stretching stretching and chamber discharge Heat-fixing Structural Supply No 115-130 chamber fixation and (prevention discharge of thermal shrinkage) Back Prevention Discharge No No chamber of air only temperature leakage control

The front chamber 6 a is a zone for preventing air from leaking out of the stretching furnace 6 through the entrance 61.

The pre-heating chamber 6 b is a zone for pre-heating the material film 12 d prior to stretching of the material film 12 d.

Each of the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e is a zone for stretching the material film 12 d in the transverse direction TD (i.e., transversely stretching the material film 12 d).

The heat-fixing chamber 6 f is a zone for fixing a structure of the material film 12 d having been stretched, so as to (i) increase a thermal stability of the material film 12 d and (ii) prevent thermal shrinkage of the material film 12 d.

The back chamber 6 g is a zone for preventing air from leaking out of the stretching furnace 6 through the exit 62.

Note that, in Embodiment 1, a width (i.e., a length in the transverse direction TD) of each of the second stretching chamber 6 d, the third stretching chamber 6 e, the heat-fixing chamber 6 f, and the back chamber 6 g is set so as to be greater than a width of each of the front chamber 6 a, the pre-heating chamber 6 b, and the first stretching chamber 6 c.

According to the stretching furnace 6, each of the front chamber 6 a, connected to the entrance 61, and the back chamber 6 g, connected to the exit 62, is configured such that air can be merely discharged therefrom. Meanwhile, each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f, which are located between the front chamber 6 a and the back chamber 6 g, is configured such that (i) air can be supplied thereto and discharged therefrom and (ii) an operating temperature therein is independently controllable. For example, a temperature in the pre-heating chamber 6 b is controlled within a range of 115° C. to 125° C. during operation. A temperature in each of the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e is controlled within a range of 100° C. to 115° C. during operation. A temperature in the heat-fixing chamber 6 f is controlled within a range of 115° C. to 130° C. during operation. By thus configuring each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f such that the operating temperature therein is independently controllable, it is possible to appropriately set an optimal temperature in accordance with a stretching condition of the material film 12 d.

An air discharge duct for discharging air is connected to a ceiling of each of the front chamber 6 a, the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, the heat-fixing chamber 6 f, and the back chamber 6 g. Each air discharge duct is connected to a collecting duct. Air, having been taken in the collecting duct from each chamber through a corresponding air discharge duct, is discharged outside by action of an air discharge fan which is connected to the collecting duct. An amount of air to be discharged from each of the front chamber 6 a, the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, the heat-fixing chamber 6 f, and the back chamber 6 g is independently controlled by use of an air discharge damper which is provided in the air discharge duct.

An air supply duct for supplying heated air and cool air is connected to the ceiling of each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f. An amount of air to be supplied to each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f is independently controlled (i) by use of an air supply damper which is provided in the air supply duct and (ii) by control of an amount of air to be caused to flow by an air supply fan which is separately attached to the ceiling of each of the chambers.

FIG. 6 is a cross-sectional view illustrating the front chamber 6 a of the stretching furnace 6. FIG. 6 illustrates a cross section of the front chamber 6 a which cross section is obtained by cutting the front chamber 6 a along a plane perpendicular to the machine direction MD. Note that the front chamber 6 a is substantially identical in main configuration to the back chamber 6 g, and therefore the front chamber 6 a will be described below as an example.

An air discharge duct 81 a through which air is discharged from the front chamber 6 a is connected to the ceiling of the front chamber 6 a (see FIG. 6). The air discharge duct 81 a is connected to a collecting duct 83. Air, having been taken in the collecting duct 83 from the front chamber 6 a through the air discharge duct 81 a, is discharged outside by action of an air discharge fan 84 which is connected to the collecting duct 83. An amount of air to be discharged from the front chamber 6 a is controlled by use of an air discharge damper 82 a which is provided in the air discharge duct 81 a.

FIG. 7 is a cross-sectional view illustrating the pre-heating chamber 6 b of the stretching furnace 6. FIG. 7 illustrates a cross section of the pre-heating chamber 6 b which cross section is obtained by cutting the pre-heating chamber 6 b along a plane perpendicular to the machine direction MD. Note that the pre-heating chamber 6 b is substantially identical in main configuration to the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f, and therefore those chambers will be exemplified by the pre-heating chamber 6 b below.

An air supply duct 71 through which heated air and cool air are supplied to heating chamber 6 b is connected to the ceiling of the pre-heating chamber 6 b (see FIG. 7). The air supply duct 71 is made up of (i) a heated air supply duct 711 for supplying heated air (at a temperature of 90° C. to 120° C.) to the pre-heating chamber 6 b and (ii) a cool air supply duct 712 for supplying, to the pre-heating chamber 6 b, ambient air in the factory.

An air supply fan 72 is also attached to the ceiling of the pre-heating chamber 6 b. An amount of air to be supplied to the pre-heating chamber 6 b is controlled (i) by use of (a) a heated air supply damper 713, which is provided in the heated air supply duct 711, and (b) a cool air supply damper 714, which is provided in the cool air supply duct 712, and (ii) by control of an amount of air to be caused to flow by the air supply fan 72, which is attached to the ceiling of the pre-heating chamber 6 b.

In the pre-heating chamber 6 b, by action of the air supply fan 72, air is (i) supplied to the pre-heating chamber 6 b through the air supply duct 71, (ii) heated, (iii) taken in a blower duct 73, and then (iv) emitted toward the material film 12 d, so that the material film 12 d is heated.

Specifically, the air having been supplied to the pre-heating chamber 6 b through the air supply duct 71 is introduced into the blower duct 73 through a hopper 74, by action of the air supply fan 72. In so doing, the air having been taken in the hopper 74 is heated by a heater 75 which is attached to the hopper 74. The air thus heated by the heater 75 is then introduced into the blower duct 73.

An end of the blower duct 73 branches into an upper nozzle duct 731 and a lower nozzle duct 732. The upper nozzle duct 731 is provided so as to be located above the material film 12 d transferred in the pre-heating chamber 6 b, and the lower nozzle duct 732 is provided so as to be located below the material film 12 d transferred in the pre-heating chamber 6 b.

A surface of the upper nozzle duct 731 which surface is to face an upper surface of the material film 12 d has a plurality of openings through which heated air is emitted toward the upper surface of the material film 12 d. A surface of the lower nozzle duct 732 which surface is to face a lower surface of the material film 12 d has a plurality of openings through which heated air is emitted toward the lower surface of the material film 12 d. This makes it possible to uniformly heat the upper surface and the lower surface of the material film 12 d.

A blower damper 733 is provided at a part where the blower duct 73 branches into the upper nozzle duct 731 and the lower nozzle duct 732. An amount of air to be emitted toward the material film 12 d is controlled by use of the blower damper 733.

An air discharge duct 81 b for discharging air from the pre-heating chamber 6 b is also connected to the ceiling of the pre-heating chamber 6 b. The air discharge duct 81 b is connected to a collecting duct 83. Air, having been taken in the collecting duct 83 from the pre-heating chamber 6 b through the air discharge duct 81 b, is discharged outside by action of an air discharge fan 84, which is connected to the collecting duct 83. An amount of air to be discharged from the pre-heating chamber 6 b is controlled by use of an air discharge damper 82 b, which is provided in the air discharge duct 81 b.

According to the film-stretching apparatus 5 configured as described above, both sides of a material film 12 d having a width W1 (e.g., a width of approximately 30 cm) are first fixed by chucks C provided on tenter rails R. By the chucks C moving on the tenter rails R in the machine direction MD, the material film 12 d is transferred into the stretching furnace 6 through the entrance 61. Note that the entrance 61 has a width which is set so that (i) the material film 12 d and (ii) the tenter rails R, each of which is provided so as to be looped, can be laid in the entrance 61. The entrance 61 has a height which is set to approximately 25 cm.

The material film 12 d moved in the machine direction MD in accordance with movement of the chucks C is transferred into the pre-heating chamber 6 b via the front chamber 6 a, and then heated in the pre-heating chamber 6 b. In the pre-heating chamber 6 b, the material film 12 d is heated to a temperature at which the material film 12 d can be sufficiently stretched. A temperature in the pre-heating chamber 6 b is approximately 115° C. to 125° C.

The material film 12 d having been pre-heated is then sequentially transferred from the pre-heating chamber 6 b to the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e. In each of the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e, the material film 12 d is stretched in the transverse direction TD while being heated. A temperature in each of the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e is approximately 100° C. to 115° C.

In a case where the material film 12 d is made of a polyethylene-based resin, there is a tendency that it is possible to more uniformly stretch the material film 12 d by stretching the material film 12 d, having been pre-heated, at a temperature lower than a temperature to which the material film 12 d has been pre-heated. This consequently makes it possible to obtain a separator original sheet 12 c which is excellent in uniformity in thickness and uniformity in phase difference. Therefore, in a case where the material film 12 d is made of a polyethylene-based resin, the temperature in each of the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e is preferably 10° C. to 25° C. lower than the temperature in the pre-heating chamber 6 b.

In the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e, the material film 12 d is stretched by increasing, in the transverse direction TD, a distance between (i) the chucks C which are located on one side and are holding one of the sides of the material film 12 d and (ii) the chucks C which are located on an opposite side and are holding the other one of the sides of the material film 12 d. That is, the chucks C which are located on the one side become distant, in the transverse direction TD, from the chucks C which are located on the opposite side, while moving in the machine direction MD, so that the material film 12 d is pulled and stretched in the transverse direction TD. This consequently causes the material film 12 d having the width W1 to be stretched so as to have a width W2 (which is, for example, substantially five times greater than the width W1). Note here that the stretching furnace 6 includes a plurality of stretching chambers (i.e., the first stretching chamber 6 c, the second stretching chamber 6 d, and the third stretching chamber 6 e), and the plurality of stretching chambers have, as a whole, a great length (in the machine direction MD). A speed at which the material film 12 d is deformed (stretched) (i.e., a strain rate [%/sec]) is (i) inversely proportional to a total length of the plurality of stretching chambers and (ii) proportional to a speed at which the material film 12 d is transferred (transferring speed). Therefore, by increasing the total length of the plurality of stretching chambers, it is possible to increase the transferring speed while maintaining the strain rate. This allows an improvement in productivity.

The material film 12 d having been stretched is then transferred into the heat-fixing chamber 6 f. In the heat-fixing chamber 6 f, the material film 12 d having been stretched is heated to a given temperature while the width W2 of the material film 12 d having been stretched is maintained. This causes an improvement in thermal stability of the material film 12 d having been stretched. A temperature in the heat-fixing chamber 6 f is approximately 115° C. to 130° C.

The material film 12 d having been subjected to heat fixation in the heat-fixing chamber 6 f is transferred into the back chamber 6 g, and then transferred out of the stretching furnace 6 through the exit 62. Note that the exit 62 has a width which is set so that (i) the separator original sheet 12 c and (ii) the tenter rails R, each of which is provided so as to be looped, can be laid in the exit 62. The exit 62 has a height which is set to approximately 25 cm.

As described above, the separator original sheet 12 c which is to be used as a base material of a separator 12 can be obtained by stretching, in the transverse direction TD, the material film 12 d in the stretching furnace 6 of the film-stretching apparatus 5.

Note, here, that a conventional film-stretching apparatus has the following problem. That is, in a case where a material film is heated, a component (i.e., a volatile component) contained in the material film, which component is in a liquid or solid state at a room temperature, is volatilized from the material film. The component thus volatilized is likely to leak out of a stretching furnace through each of an entrance and an exit of the stretching furnace, be condensed and deposited outside the stretching furnace, and then drop or fall on the material film. This may cause damage to the material film.

Examples of the component, which is contained in the material film and which is in a liquid or solid state at a room temperature, include: polyolefin processing aids; antioxidants and denatured antioxidants; polyolefin plasticizing agents; subsidiary substances each contained in a petroleum-derived polyolefin plasticizing agent; cleaning solvents each used to clean a film or a roll of the film prior to the stretching step; stabilizing agents each of which is added to a cleaning agent which prevents propagation, rot, oxidization, and/or decomposition of an organism; antistatic agents; and the like. Each of those components can be added in any of the kneading step S1, the sheet-forming step S2, the removing step S3, and the stretching step S4.

Examples of the polyolefin processing aids include: higher fatty acids such as lauric acid and stearic acid; and metallic salts thereof. Examples of the antioxidants and the denatured antioxidants include: antioxidants such as a phenolic antioxidant and a phosphorous antioxidant; and denatured antioxidants such as a denatured phenolic antioxidant and a denatured phosphorous antioxidant. Examples of the polyolefin plasticizing agents include: hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. Examples of the subsidiary substances each contained in a petroleum-derived polyolefin plasticizing agent include subsidiary substances each contained in a petroleum-derived polyolefin plasticizing agent such as liquid paraffins such as an olefin compound, a naphthene compound, and an (polynuclear) aromatic compound. Examples of the cleaning solvents each used to clean a film or a roll of the film prior to the stretching step include: water; alcohols such as methanol, ethanol, and isopropanol; halogenated hydrocarbons such as methylene chloride; and hydrocarbons, such as hexane and heptane, each of which is in a liquid state at a room temperature. Examples of the stabilizing agents, each of which is added to a cleaning agent which prevents propagation, rot, oxidization, and/or decomposition of an organism, include alcohols such as methanol and isopropanol. Examples of the antistatic agents include: water; and alcohols such as methanol, ethanol, and isopropanol.

In view of the above problem, the film-stretching apparatus 5 in accordance with Embodiment 1 is configured such that an airflow in each of the entrance 61 and the exit 62 of the stretching furnace 6 is controlled so as to be directed toward an inside of the stretching furnace 6. This makes it possible to prevent the volatile component from leaking out of the stretching furnace 6 through each of the entrance 61 and the exit 62 of the stretching furnace 6, and consequently allows a reduction in damage to the material film.

Specifically, according to the film-stretching apparatus 5, the front chamber 6 a for preventing air from leaking out of the stretching furnace 6 through the entrance 61 is provided on the entrance 61 side of the stretching furnace 6. A pressure in the front chamber 6 a connected to the entrance 61 is controlled so as to be sufficiently lower than an atmospheric pressure (i.e., so as to be a negative pressure). This causes, in the entrance 61, an airflow directed toward the inside of the stretching furnace 6, and consequently prevents the volatile component from leaking out of the stretching furnace 6 through the entrance 61. Similarly, the back chamber 6 g for preventing air from leaking out of the stretching furnace 6 through the exit 62 is provided on an exit 62 side of the stretching furnace 6. A pressure in the back chamber 6 g connected to the exit 62 is controlled so as to be sufficiently lower than the atmospheric pressure (i.e., so as to be a negative pressure). This causes, in the exit 62, an airflow directed toward the inside of the stretching furnace 6, and consequently prevents the volatile component from leaking out of the stretching furnace 6 through the exit 62.

FIG. 8 is a graph showing (i) an amount of air (air supply amount), per unit volume, supplied to each chamber of the stretching furnace 6 and (ii) an amount of air (air discharge amount), per unit volume, discharged from each chamber of the stretching furnace 6. As shown in FIG. 8, according to the film-stretching apparatus 5, each of the front chamber 6 a and the back chamber 6 g is configured such that (i) air is not supplied to the each of the front chamber 6 a and the back chamber 6 g and (ii) an amount of air, per unit volume, to be discharged from the each of the front chamber 6 a and the back chamber 6 g is larger than an amount of air, per unit volume, to be discharged from each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f. A pressure in each of the front chamber 6 a and the back chamber 6 g is thus controlled so as to be relatively lower than a pressure in each of the pre-heating chamber 6 b, the first stretching chamber 6 c, the second stretching chamber 6 d, the third stretching chamber 6 e, and the heat-fixing chamber 6 f.

A difference between a pressure in the stretching furnace 6 (i.e., the pressure in each of the front chamber 6 a and the back chamber 6 g) and the atmospheric pressure is preferably not smaller than 2 Pa, more preferably not smaller than 5 Pa, and still more preferably not smaller than 10 Pa. In a case where such a pressure difference is smaller than 2 Pa, it is difficult to prevent the volatile component from leaking out of the stretching furnace 6 through each of the entrance 61 and the exit 62, because the pressure in the stretching furnace 6 is substantially equal to the atmospheric pressure. By making the above-described difference between the pressure in the stretching furnace 6 and the atmospheric pressure, it is possible to cause an airflow which has an airflow rate of not less than 0.5 m/sec and which is directed toward the inside of the stretching furnace 6. Particularly, the airflow in the entrance 61 preferably has an airflow rate of not smaller than 1.0 m/sec because the volatile component is volatilized in a larger amount in the pre-heating chamber 6 b of the stretching furnace 6.

By thus (i) providing, in the stretching furnace 6, the front chamber 6 a, which is connected to the entrance 61, and the back chamber 6 g, which is connected to the exit 62, and (ii) controlling the pressure in each of the front chamber 6 a and the back chamber 6 g so as to be lower than the atmospheric pressure (i.e., so as to be a negative pressure), it is possible to suitably control an airflow in each of the entrance 61 and the exit 62 so as to be directed toward the inside of the stretching furnace 6.

According to the film-stretching apparatus 5, an airflow in each of the entrance 61 and the exit 62 is controlled so as to be directed toward the inside of the stretching furnace 6. Furthermore, a gas in the stretching furnace 6 is controlled so as to be discharged outside the stretching furnace 6. Specifically, according to the film-stretching apparatus 5, each of the front chamber 6 a and the back chamber 6 g is configured such that air is merely discharged therefrom. Meanwhile, each of the pre-heating chamber 6 b, the first stretching chamber 6 c, and the heat-fixing chamber 6 f is configured such that an amount of air, per unit volume, to be discharged therefrom is larger than an amount of air, per unit volume, to be supplied thereto. This makes it possible to discharge, outside the stretching furnace 6 through the air discharge duct, (i) external air having entered the stretching furnace 6 and (ii) the volatile component having been volatilized in the stretching furnace 6. Consequently, it is possible to prevent the volatile component from being condensed and deposited in the stretching furnace 6 due to an effect of external air having entered the stretching furnace 6 (e.g., due to a decrease in temperature in the stretching furnace 6).

Particularly, an amount of the volatile component which is volatilized by heating the material film 12 d in the stretching furnace 6 is relatively larger on the entrance 61 side of the stretching furnace 6 than on the exit 62 side of the stretching furnace 6. In other words, most part of the volatile component contained in the material film 12 d is volatilized by a time when the material film 12 d starts to be stretched in the first stretching chamber 6 c of the stretching furnace 6. In view of the circumstances, it is preferable to control an amount of air to be discharged from at least part of the stretching furnace 6, which part extends from the entrance 61 to a location at which the material film 12 d starts to be stretched (i.e., which part extends from the entrance 61 to a location at which a distance between opposing tenter rails R starts to be increased), so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace 6, by configuring (i) the front chamber 6 a such that air is merely discharged therefrom and (ii) each of the pre-heating chamber 6 b, which is located subsequent to the front chamber 6 a, and the first stretching chamber 6 c, which is located subsequent to the pre-heating chamber 6 b, such that an amount of air, per unit volume, to be discharged therefrom is larger than an amount of air, per unit volume, to be supplied thereto. This makes it possible to (i) control a pressure in the part of the stretching furnace so as to be a negative pressure so that the volatile component is prevented from leaking out of the stretching furnace 6 through the entrance 61 and (ii) discharge, outside the stretching furnace 6 through the air discharge duct, (a) external air having entered the stretching furnace 6 and (b) the volatile component having been volatilized in the stretching furnace 6.

FIG. 9A is a cross-sectional view illustrating an airflow AF in the entrance 61 of the stretching furnace 6. FIG. 9B is a cross-sectional view illustrating an airflow AF in the exit of the stretching furnace 6. Each of FIGS. 9A-9B illustrates a cross section of the film-stretching apparatus 5 which cross section is obtained by cutting the film-stretching apparatus 5 along a plane perpendicular to the transverse direction TD.

By controlling the pressure in the front chamber 6 a, connected to the entrance 61, so as to be a negative pressure, it is possible to control the airflow AF in the entrance 61 so as to be directed toward the inside of the stretching furnace 6 (see FIG. 9A). Similarly, by controlling the pressure in the back chamber 6 g, connected to the exit 62, so as to be a negative pressure, it is possible to control the airflow AF in the exit 62 so as to be directed toward the inside of the stretching furnace 6 (see FIG. 9B). This prevents the volatile component VC from leaking out of the stretching furnace 6 through each of the entrance 61 and the exit 62, and consequently allows a reduction in damage to the material film 12 d and the separator original sheet 12 c which damage is caused by leakage of the volatile component VC from the stretching furnace 6 through each of the entrance 61 and the exit 62.

(Summary of Film-Stretching Apparatus)

As described above, the film-stretching apparatus 5 in accordance with Embodiment 1 includes the stretching furnace 6 having (i) the entrance 61 through which the material film 12 d is transferred into the stretching furnace 6 and (ii) the exit 62 through which the material film 12 d having been stretched (separator original sheet 12 c) is transferred out of the stretching furnace 6. The airflow AF in each of the entrance 61 and the exit 62 is controlled so as to be directed toward the inside of the stretching furnace 6. Furthermore, a gas in the stretching furnace 6 is controlled so as to be discharged outside the stretching furnace 6.

According to the film-stretching apparatus 5, the airflow AF in each of the entrance 61 and the exit 62 of the stretching furnace 6 is controlled so as to be directed toward the inside of the stretching furnace 6. This makes it possible to prevent the volatile component VC, having been volatilized from the material film 12 d, from leaking out the stretching furnace 6 through each of the entrance 61 and the exit 62.

Furthermore, according to the film-stretching apparatus 5, a gas in the stretching furnace 6 is controlled so as to be discharged outside the stretching furnace 6. This makes it possible to discharge, outside the stretching furnace 6, (i) external air having entered the stretching furnace 6 and (ii) the volatile component having been volatilized in the stretching furnace 6. It is therefore possible to prevent the volatile component from being condensed and deposited in the stretching furnace 6 due to an effect of external air having entered the stretching furnace 6.

Embodiment 1 therefore makes it possible to provide a film-stretching apparatus 5 which allows a reduction in damage to the film which damage is caused by leakage of the volatile component VC from the stretching furnace 6 through each of the entrance 61 and the exit 62.

Embodiment 1 has discussed a configuration in which the stretching furnace 6 includes both of (i) the front chamber 6 a for preventing air from leaking out of the stretching furnace 6 through the entrance 61 and (ii) the back chamber 6 g for preventing air from leaking out of the stretching furnace 6 through the exit 62. The present invention is, however, not limited to such a configuration. An amount of the volatile component VC which is volatilized by heating the material film 12 d in the stretching furnace 6 is relatively larger on the entrance 61 side of the stretching furnace 6 than on the exit 62 side of the stretching furnace 6. It follows that, by controlling the airflow AF at least in the entrance 61 of the stretching furnace 6 so as to be directed toward the inside of the stretching furnace 6, it is possible to sufficiently reduce damage to a film which damage is caused by leakage of the volatile component VC from the stretching furnace 6 through the entrance 61 of the stretching furnace 6. The back chamber 6 g is therefore not essential, and can be alternatively omitted.

Embodiment 1 has discussed a configuration in which the temperature in each of the front chamber 6 a and the back chamber 6 g of the stretching furnace 6 is not controlled. The present invention is, however, not limited to such a configuration. Alternatively, the present invention can be configured such that the temperature in each of the front chamber 6 a and the back chamber 6 g is also independently controllable. By heating the front chamber 6 a and the back chamber 6 g, it is possible to more effectively prevent the volatile component VC, having been volatilized from the material film, from leaking out of the stretching furnace 6 through each of the entrance 61 and the exit 62. That is, by heating each of the front chamber 6 a and the back chamber 6 g, a density of air in each of the front chamber 6 a and the back chamber 6 g is decreased. This causes (i) a reduction in pressure loss caused by discharging air from the stretching furnace 6 and (ii) an increase in amount of air to be discharged through the air discharge duct. It is therefore possible to more effectively prevent the volatile component VC from leaking out of the stretching furnace 6 through each of the entrance 61 and the exit 62. By causing the temperature to be independently controllable in each of the front chamber 6 a and the back chamber 6 g, it is possible to use the front chamber 6 a and the back chamber 6 g to heat the material film 12 d in a case where time for pre-heating or heat fixation is insufficient.

Embodiment 1 has discussed the film-stretching apparatus 5 as an example configuration of a uniaxial stretching apparatus which stretches the material film 12 d in the width direction (i.e., the transverse direction TD). The present invention is, however, not limited to such a configuration. Alternatively, the film-stretching apparatus in accordance with an embodiment of the present invention can be a simultaneous biaxial stretching apparatus which stretches the material film 12 d simultaneously in a longitudinal (length) direction (i.e., the machine direction MD) and the width direction (i.e., the transverse direction TD).

Note that the film-stretching apparatus 5 in accordance with Embodiment 1 can be rephrased as follows. That is, the film-stretching apparatus 5 includes the stretching furnace 6 having (i) the entrance 61 through which the material film 12 d is transferred into the stretching furnace 6 and (ii) the exit 62 through which the material film 12 d having been stretched (i.e., the separator original sheet 12 c) is transferred out of the stretching furnace 6, an amount of air to be discharged from at least part of the stretching furnace 6 being controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace 6, the at least part of the stretching furnace 6 extending from the entrance 61 to a location at which the material film 12 d starts to be stretched.

An amount of the volatile component VC which is volatilized by heating the material film 12 d in the stretching furnace 6 is relatively larger on the entrance 61 side of the stretching furnace 6 than on the exit 62 side of the stretching furnace 6. In other words, most part of the volatile component VC contained in the material film 12 d is volatilized by a time when the material film 12 d starts to be stretched in the first stretching chamber 6 c of the stretching furnace 6. According to the film-stretching apparatus 5, an amount of air to be discharged from part of the stretching furnace 6, which part extends from the entrance 61 to the location at which the material film 12 d starts to be stretched (i.e., which part extends from the entrance 61 to a location at which the distance between the opposing tenter rails R starts to be increased), is controlled so as to be larger than an amount of air to be supplied to the part of the stretching furnace 6. This makes it possible to (i) control a pressure in the part of the stretching furnace 6 so as to be a negative pressure so that the volatile component VC is prevented from leaking out of the stretching furnace 6 through the entrance 61 of the stretching furnace 6 and (ii) discharge, outside the stretching furnace 6, (a) external air having entered the stretching furnace 6 and (b) the volatile component VC having been volatilized in the stretching furnace 6.

Since the amount of air to be discharged from the part of the stretching furnace 6, which part extends from the entrance 61 to the location at which the material film 12 d starts to be stretched, is controlled so as to be larger than the amount of air to be supplied to the part of the stretching furnace 6, it is possible to suitably reduce, for example, damage to the material film 12 d which damage is caused by leakage of the volatile component VC from the stretching furnace 6 through the entrance 61.

Embodiment 2

The following description will discuss Embodiment 2 of the present invention with reference to FIGS. 10 and 11A-11B. In Embodiment 2, described will be an example of a method (film production method) of producing a separator for a lithium ion secondary battery in which method a separator original sheet obtained in Embodiment 1 is used as a base material.

(Method of Producing Separator)

FIG. 10 is a flowchart schematically showing a method of producing a separator. The separator is configured such that a functional layer is stacked on a separator original sheet 12 c which serves as a base material. Examples of the functional layer encompass a heat-resistant layer and an adhesive layer.

The functional layer is stacked on the separator original sheet 12 c by (i) coating the separator original sheet 12 c with a coating material (a material) or the like corresponding to the functional layer and then (ii) drying the coating material.

FIG. 10 shows, as an example, a flow of producing a heat-resistant separator 12 a in a case where the functional layer is a heat-resistant layer 4. The flow shown in FIG. 10 is an example of a flow in which a wholly aromatic polyamide (aramid resin) is employed as a material of the heat-resistant layer 4 and such a heat-resistant layer 4 is stacked on the separator original sheet 12 c.

The flow includes a first inspecting step S11, a coating step S12, a depositing step S13, a cleaning step S14, a drying step S15, a second inspecting step S16, and a slitting step S17.

The steps S11 through S17, which follow the above-described flow of producing the separator original sheet 12 c, will be described in order below.

(First Inspecting Step S11)

The first inspecting step S11 is a step of inspecting the separator original sheet 12 c, obtained in Embodiment 1, before carrying out the subsequent coating step.

(Coating Step S12)

The coating step S12 is a step of coating, with a coating material (a material) to be the heat-resistant layer 4, the separator original sheet 12 c having been inspected in the first inspecting step S11. In the coating step S12, such coating can be carried out with respect to only one of surfaces of the separator original sheet 12 c or can alternatively be carried with respect to each of the surfaces of the separator original sheet 12 c.

For example, in the coating step S12, the separator original sheet 12 c is coated with a solution obtained by dissolving an aramid in NMP (N-methyl-pyrrolidone), as the coating material to be the heat-resistant layer 4. Note that the heat-resistant layer 4 is not limited to an aramid heat-resistant layer. For example, the separator original sheet 12 c can alternatively be coated with a suspension of alumina, carboxymethyl cellulose, and water, as the coating material to be the heat-resistant layer 4.

A method of coating the separator original sheet 12 c with the coating material is not particularly limited as long as uniform wet coating can be performed with respect to the separator original sheet 12 c by the method, and various methods can be employed.

For example, it is possible to employ any of methods such as a capillary coating method, a slit die coating method, a spray coating method, a dip coating method, a roll coating method, a screen printing method, a flexo printing method, a bar coater method, a gravure coater method, and a die coater method.

The material of the heat-resistant layer 4 with which material the separator original sheet 12 c is coated has a film thickness that can be controlled by adjusting a thickness of a coating wet film and a solid-content concentration of the coating material.

(Depositing Step S13)

The depositing step S13 is a step of solidifying the coating material with which the separator original sheet 12 c has been coated in the coating step S12. In a case where the coating material is an aramid coating material, for example, water vapor is applied to a coated surface so that an aramid is solidified by humidity deposition. This provides a heat-resistant separator original sheet 12 b (see FIGS. 11A-11B) in which the heat-resistant layer 4 is formed on the separator original sheet 12 c.

(Cleaning Step S14)

The cleaning step S14 is a step of cleaning the heat-resistant separator original sheet 12 b obtained by solidifying the coating material in the depositing step S13. In a case where the heat-resistant layer 4 is an aramid heat-resistant layer, for example, water, an aqueous solution, or an alcohol-based solution is suitably used as a cleaning liquid.

Note that, in the cleaning step S14, multistage cleaning can be carried out, that is, cleaning can be carried out a plurality of times in order to enhance a cleaning effect.

Note also that, after the cleaning step S14, a water removing step can be carried out which is a step of removing water from the heat-resistant separator original sheet 12 b having been cleaned in the cleaning step S14. A purpose of removing the water is to remove water or the like, adhering to the heat-resistant separator original sheet 12 b, before carrying out the subsequent drying step S15 so that drying can be carried out more easily and insufficient drying can be prevented.

(Drying Step S15)

The drying step S15 is a step of drying the heat-resistant separator original sheet 12 b having been cleaned in the cleaning step S14. A method of drying the heat-resistant separator original sheet 12 b is not particularly limited, and, for example, it is possible to employ various methods such as (i) a method in which the heat-resistant separator original sheet 12 b is brought into contact with a heated roller or (ii) a method in which hot air is blown onto the heat-resistant separator original sheet 12 b.

(Second Inspecting Step S16)

The second inspecting step S16 is a step of inspecting the heat-resistant separator original sheet 12 b having been dried in the drying step S15. In inspection, a defect is marked as appropriate, and it is therefore possible to efficiently prevent the heat-resistant separator original sheet 12 b from having a defect.

(Slitting Step S17)

The slitting step S17 is a step of slitting (cutting) the heat-resistant separator original sheet 12 b, having been inspected in the second inspecting step S16, into pieces each having a predetermined product width. Specifically, in the slitting step S17, the heat-resistant separator original sheet 12 b is slit into pieces each having a product width which is a width suitable for a product, such as a lithium ion secondary battery 1, to which each of the pieces is to be applied.

Each of FIGS. 11A-11B is a diagram schematically illustrating an example of the slitting step S17. The slitting step S17 is carried out by use of a slitting apparatus 9 which slits the heat-resistant separator original sheet 12 b (see FIGS. 11A-11B).

The slitting apparatus 9 includes a wind-off roller 91, rollers 92 through 95, and a plurality of take-up rollers 96. Each of the wind-off roller 91, the rollers 92 through 95, and the plurality of take-up rollers 96 has a columnar shape, and is rotatably supported. The slitting apparatus 9 also includes a plurality of blades (not illustrated). A roll 10 is fitted to the wind-off roller 91. The roll 10 is a roll of the heat-resistant separator original sheet 12 b which roll is obtained by winding the heat-resistant separator original sheet 12 b on an outer peripheral surface of a core 97 so as to be layered.

In order to increase productivity, the heat-resistant separator original sheet 12 b is usually produced so as to have a width that is not smaller than the product width. After the heat-resistant separator original sheet 12 b is produced, the heat-resistant separator original sheet 12 b is slit into heat-resistant separators 12 a each having the product width.

Specifically, in the slitting step S17, the heat-resistant separator original sheet 12 b is wound off from the core 97 to a path U or a path L. The heat-resistant separator original sheet 12 b thus wound off is transferred to the roller 94 via the rollers 92 and 93. In a step of transferring the heat-resistant separator original sheet 12 b, the heat-resistant separator original sheet 12 b is slit substantially parallel to the machine direction MD. As a result, a plurality of heat-resistant separators 12 a, into which the heat-resistant separator original sheet 12 b has been slit so as to have the product width, are produced.

The plurality of heat-resistant separators 12 a thus produced are wound on respective cores 98 fitted on the take-up rollers 96.

Supplemental Notes

A film-stretching apparatus in accordance with an aspect of the present invention is a film-stretching apparatus for stretching a film, including: a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, an airflow in the entrance being controlled so as to be directed toward an inside of the stretching furnace, a gas in the stretching furnace being controlled so as to be discharged outside the stretching furnace.

An amount of a volatile component which is volatilized by heating the film in the stretching furnace is relatively larger on an entrance side of the stretching furnace than on an exit side of the stretching furnace. According to the above configuration, since the airflow in the entrance of the stretching furnace is controlled so as to be directed toward the inside of the stretching furnace, the volatile component is prevented from leaking out of the stretching furnace through the entrance of the stretching furnace. This allows a reduction in damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the entrance of the stretching furnace.

According to the above configuration, since the gas in the stretching furnace is controlled so as to be discharged outside the stretching furnace, it is possible to discharge, outside the stretching furnace, (i) external air having entered the stretching furnace and (ii) the volatile component having been volatilized in the stretching furnace. This makes it possible to prevent the volatile component from being condensed and deposited in the stretching furnace due to an effect of external air having entered the stretching furnace (e.g., a decrease in temperature in the stretching furnace).

The above configuration therefore makes it possible to realize a film-stretching apparatus which suitably allows a reduction in, for example, damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the entrance of the stretching furnace.

The film-stretching apparatus in accordance with an aspect of the present invention can be configured such that a pressure in the stretching furnace is controlled so as to be a negative pressure.

According to the above configuration, since the pressure in the stretching furnace is controlled so as to be a negative pressure, it is possible to suitably control the airflow in the entrance so as to be directed toward the inside of the stretching furnace. This makes it possible to easily prevent the volatile component, having been volatilized from the film, from leaking out of the stretching furnace through the entrance of the stretching furnace.

The film-stretching apparatus in accordance with an aspect of the present invention can be configured such that: the stretching furnace is divided, along a direction in which the film is transferred, into a plurality of air amount control zones in each of which an amount of air is independently controllable; and a pressure in one of the plurality of air amount control zones which one is connected to the entrance is controlled so as to be a negative pressure.

According to the above configuration, since at least the pressure in the one of the plurality of air amount control zones, which one is connected to the entrance, is controlled so as to be a negative pressure, it is possible to prevent the volatile component, having been volatilized from the film, from leaking out of the stretching furnace through the entrance. Furthermore, since a pressure in each of the others of the plurality of air amount control zones is independently controlled, it is possible to control the pressure in each of the others of the plurality of air amount control zones so as to be an optimal pressure, depending on contents of a process to be carried out with respect to the film in the stretching furnace.

The film-stretching apparatus in accordance with an aspect of the present invention can be configured such that an airflow in the exit is controlled so as to be directed toward the inside of the stretching furnace.

According to the above configuration, since the airflow in the exit of the stretching furnace is controlled so as to be directed toward the inside of the stretching furnace, the volatile component, having been volatilized from the film, is prevented from leaking out of the stretching furnace through the exit of the stretching furnace. This allows a reduction in damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the exit of the stretching furnace.

The film-stretching apparatus in accordance with an aspect of the present invention can be configured such that a pressure in the stretching furnace is controlled so as to be a negative pressure.

According to the above configuration, since the pressure in the stretching furnace is controlled so as to be a negative pressure, it is possible to suitably control the airflow in the exit so as to be directed toward the inside of the stretching furnace. This makes it possible to easily prevent the volatile component from leaking out of the stretching furnace through the exit of the stretching furnace.

A film-stretching apparatus in accordance with an aspect of the present invention is a film-stretching apparatus for stretching a film, including: a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, an amount of air to be discharged from at least part of the stretching furnace being controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace, the at least part of the stretching furnace extending from the entrance to a location at which the film starts to be stretched.

An amount of the volatile component which is volatilized by heating the film in the stretching furnace is relatively larger on the entrance side of the stretching furnace than on the exit side of the stretching furnace. In other words, most part of the volatile component contained in the film is volatilized by a time when the film starts to be stretched in the stretching furnace. According to the above configuration, an amount of air to be discharged from the at least part of the stretching furnace, which part extends from the entrance to the location at which the film starts to be stretched, is controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace. This makes it possible to (i) control a pressure in the at least part of the stretching furnace so as to be a negative pressure so that the volatile component is prevented from leaking out of the stretching furnace through the entrance of the stretching furnace and (ii) discharge, outside the stretching furnace, (a) external air having entered the stretching furnace and (b) the volatile component having been volatilized in the stretching furnace.

The above configuration therefore makes it possible to realize a film-stretching apparatus which suitably allows a reduction in, for example, damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the entrance of the stretching furnace.

A method of producing a film in accordance with an aspect of the present invention includes: a film stretching step of stretching a film, the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, in the film stretching step, an airflow in the entrance being controlled so as to be directed toward an inside of the stretching furnace, and a gas in the stretching furnace being controlled so as to be discharged outside the stretching furnace.

According to the above method, since the airflow in the entrance of the stretching furnace is controlled, in the film stretching step, so as to be directed toward the inside of the stretching furnace, the volatile component having been volatilized from the film is prevented from leaking out of the stretching furnace through the entrance of the stretching furnace. This allows a reduction in damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the entrance of the stretching furnace.

According to the above method, since the gas in the stretching furnace is controlled, in the film stretching step, so as to be discharged outside the stretching furnace, it is possible to discharge, outside the stretching furnace, (i) external air having entered the stretching furnace and (ii) the volatile component having been volatilized in the stretching furnace. This makes it possible to prevent the volatile component from being condensed and deposited in the stretching furnace due to an effect of external air having entered the stretching furnace (e.g., a decrease in temperature in the stretching furnace).

The above method therefore makes it possible to realize a method of producing a film which method suitably allows a reduction in, for example, damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the entrance of the stretching furnace.

A method of producing a film in accordance with an aspect of the present invention includes: a film stretching step of stretching a film, the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, in the film stretching step, an amount of air to be discharged from at least part of the stretching furnace being controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace, the at least part of the stretching furnace extending from the entrance to a location at which the film starts to be stretched.

An amount of the volatile component which is volatilized by heating the film in the stretching furnace is relatively larger on the entrance side of the stretching furnace than on the exit side of the stretching furnace. In other words, most part of the volatile component contained in the film is volatilized by a time when the film starts to be stretched in the stretching furnace. According to the above method, in the film stretching step, an amount of air to be discharged from the at least part of the stretching furnace, which part extends from the entrance to the location at which the film starts to be stretched, is controlled so as to be larger than an amount of air to be supplied to the at least part of the stretching furnace. This makes it possible to (i) control a pressure in the at least part of the stretching furnace so as to be a negative pressure so that the volatile component is prevented from leaking out of the stretching furnace through the entrance of the stretching furnace and (ii) discharge, outside the stretching furnace, (a) external air having entered the stretching furnace and (b) the volatile component having been volatilized in the stretching furnace.

The above method therefore makes it possible to realize a method of producing a film which method suitably allows a reduction in, for example, damage to the film which damage is caused by leakage of the volatile component from the stretching furnace through the entrance of the stretching furnace.

The present invention is not limited to the embodiments, but can be altered by a skilled person in the art within the scope of the claims. An embodiment derived from a proper combination of technical means each disclosed in a different embodiment is also encompassed in the technical scope of the present invention.

REFERENCE SIGNS LIST

-   5: Film-stretching apparatus -   6: Stretching furnace -   61: Entrance -   62: Exit -   6 a: Front chamber (air amount control zone) -   6 g: Back chamber (air amount control zone) -   12 c: Separator original sheet (film) -   12 d: Material film (film) -   S4: Stretching step (film stretching step) -   AF: Airflow -   MD: Machine direction 

The invention claimed is:
 1. A method of producing a film, the method comprising: a film stretching step of stretching a film, the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, in the film stretching step, an airflow in the entrance being controlled so as to be directed toward an inside of the stretching furnace, and at least a part of a gas in the stretching furnace being controlled so as to be discharged outside the stretching furnace from a part of the stretching furnace which extends from the entrance to a location before the film starts to be stretched in a direction in which the film is transferred or in a width direction, the gas including air which has entered the stretching furnace from the entrance.
 2. The method as set forth in claim 1, further comprising controlling a pressure in the stretching furnace to be a negative pressure.
 3. The method as set forth in claim 2, wherein a difference between the pressure in the stretching furnace and atmospheric pressure is not smaller than 2 Pa.
 4. The method as set forth in claim 3, wherein the airflow directed toward the inside of the stretching furnace has an airflow rate of not less than 0.5 m/sec.
 5. The method as set forth in claim 1, further comprising, in the film stretching step, controlling an airflow in the exit so as to be directed toward the inside of the stretching furnace.
 6. The method as set forth in claim 5, further comprising controlling a pressure in the stretching furnace to be a negative pressure.
 7. The method as set forth in claim 6, wherein a difference between the pressure in the stretching furnace and atmospheric pressure is not smaller than 2 Pa.
 8. The method as set forth in claim 7, wherein the airflow directed toward the inside of the stretching furnace has an airflow rate of not less than 0.5 m/sec.
 9. The method as set forth in claim 1, wherein the stretching furnace is divided, along a direction in which the film is transferred, into a plurality of air amount control zones, and wherein the method further comprises independently controlling an amount of air in each of the air amount control zones.
 10. The method as set forth in claim 9, further comprising controlling a pressure in one of the plurality of air amount control zones which is connected to the entrance to be a negative pressure.
 11. The method as set forth in claim 9, further comprising independently controlling an operating temperature of each of the air amount control zones.
 12. The method as set forth in claim 9, wherein the plurality of air amount control zones includes a pre-heating chamber and at least a first stretching chamber.
 13. The method as set forth in claim 12, wherein a temperature in the pre-heating chamber is controlled within a range of 115° C. to 125° C. and wherein a temperature in the at least first stretching chamber is controlled within a range of 100° C. to 115° C.
 14. The method as set forth in claim 12, wherein the plurality of air amount control zones includes a heat-fixing chamber in which the film which has been stretched is heated while a width of the film which has been stretched is maintained.
 15. The method as set forth in claim 14, wherein a temperature in the heat-fixing chamber is controlled within a range of 115° C. to 130° C.
 16. The method as set forth in claim 1, further comprising pre-heating the film in the stretching furnace prior to stretching the film.
 17. The method as set forth in claim 16, wherein the film comprises a polyethylene-based resin.
 18. The method as set forth in claim 17, wherein the film stretching step is carried out at a temperature which is lower than a temperature to which the film has been pre-heated.
 19. The method as set forth in claim 1, wherein the film is uniaxially or biaxially stretched in the stretching furnace.
 20. A method of producing a film, the method comprising: a film stretching step of stretching a film, the film stretching step being carried out by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace, in the film stretching step, an amount of air to be discharged outside the stretching furnace from a part of the stretching furnace being controlled so as to be larger than an amount of air to be supplied to the part of the stretching furnace from the outside of the stretching furnace, the part of the stretching furnace extending from the entrance to a location before the film starts to be stretched in a direction in which the film is transferred or in a width direction. 